Add WSTRING support for structures (#293)

* Add WSTRING support for dict_from-bytes

* Add helper function to find null-terminator in WSTRING

* Add test for find_wstring_null_terminator function

* Use find_wstring_null_terminator function in adsSumRead

* Add WSTRING support for size_of_structure

* Fix size_of_structure.

Broke it in the commit before by modifying the conditions.

* Add WSTRING support for bytes_from_dict

* Fix bytes_from_dict

Array size was not calculated correctly

* Use length of bytearray

* Improve performance on adding remaining bytes

This may not be necessary but replacing the for loop with the extend statement speeds speeds it up by factor 5

* Fix size_of_structure for multiple fields

* Add test for read/write WSTRING struct

* Add test for read/write WSTRING array struct

* Add Changelog entry

* Fix str_len

- str_len now is always the number of characters without null-terminator
- n_bytes is the number of bytes and differs for STRING or WSTRING

* Add tests for size_of_structure for WSTRING

* Add WSTRING to test_dict_from_bytes

* Add WSTRING to test_bytes_from_dict

* Fix WSTRING length for bytes_from_strings

* Add comments

* Remove redundant comment

Author: stlehmann

CHANGELOG.md

@@ -4,9 +4,10 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
-## 3.3.10
+## 3.3.10 [unreleased]
 
 ### Added
+* [#293](https://github.com/stlehmann/pyads/pull/2939) Support WSTRINGS in structures
 
 ### Changed
 * [#292](https://github.com/stlehmann/pyads/pull/292) Improve performance of get_value_from_ctype_data for arrays

pyads/ads.py

@@ -31,6 +31,7 @@
     PLCTYPE_REAL,
     PLCTYPE_SINT,
     PLCTYPE_STRING,
+    PLCTYPE_WSTRING,
     PLCTYPE_TIME,
     PLCTYPE_TOD,
     PLCTYPE_UDINT,
@@ -62,7 +63,7 @@
     AmsAddr,
     SAmsNetId,
 )
-from .utils import platform_is_linux
+from .utils import platform_is_linux, find_wstring_null_terminator
 
 # custom types
 StructureDef = Tuple[
@@ -259,9 +260,14 @@ def size_of_structure(structure_def: StructureDef) -> int:
 
         if plc_datatype == PLCTYPE_STRING:
             if str_len is not None:
-                num_of_bytes += (str_len + 1) * size
+                num_of_bytes += (str_len + 1) * size  # STRING uses 1 byte per character + null-terminator
             else:
                 num_of_bytes += (PLC_DEFAULT_STRING_SIZE + 1) * size
+        elif plc_datatype == PLCTYPE_WSTRING:
+            if str_len is not None:
+                num_of_bytes += 2 * (str_len + 1) * size  # WSTRING uses 2 bytes per character + null-terminator
+            else:
+                num_of_bytes += (PLC_DEFAULT_STRING_SIZE + 1) * 2 * size
         elif plc_datatype not in DATATYPE_MAP:
             raise RuntimeError("Datatype not found")
         else:
@@ -306,6 +312,7 @@ def dict_from_bytes(
                 var, plc_datatype, size = item  # type: ignore
                 str_len = None
             except ValueError:
+                # str_len is the numbers of characters without null-terminator
                 var, plc_datatype, size, str_len = item  # type: ignore
 
             var_array = []
@@ -319,6 +326,14 @@ def dict_from_bytes(
                         .decode("utf-8")
                     )
                     index += str_len + 1
+                elif plc_datatype == PLCTYPE_WSTRING:
+                    if str_len is None:  # if no str_len is given use default size
+                        str_len = PLC_DEFAULT_STRING_SIZE
+                    n_bytes = 2 * (str_len + 1)  # WSTRING uses 2 bytes per character + null-terminator
+                    a = bytearray(byte_list[index: (index + n_bytes)])
+                    null_idx = find_wstring_null_terminator(a)
+                    var_array.append(a[:null_idx].decode("utf-16-le"))
+                    index += n_bytes
                 elif plc_datatype not in DATATYPE_MAP:
                     raise RuntimeError("Datatype not found. Check structure definition")
                 else:
@@ -392,12 +407,23 @@ def bytes_from_dict(
                         str_len = PLC_DEFAULT_STRING_SIZE
                     if size > 1:
                         byte_list += list(var[i].encode("utf-8"))
-                        remaining_bytes = str_len + 1 - len(var[i])
+                        remaining_bytes = str_len + 1 - len(var[i])  # 1 byte a character plus null-terminator
                     else:
                         byte_list += list(var.encode("utf-8"))
-                        remaining_bytes = str_len + 1 - len(var)
-                    for byte in range(remaining_bytes):
-                        byte_list.append(0)
+                        remaining_bytes = str_len + 1 - len(var)  # 1 byte a character plus null-terminator
+                    byte_list.extend(remaining_bytes * [0])
+                elif plc_datatype == PLCTYPE_WSTRING:
+                    if str_len is None:
+                        str_len = PLC_DEFAULT_STRING_SIZE
+                    if size > 1:
+                        encoded = list(var[i].encode("utf-16-le"))
+                        byte_list += encoded
+                        remaining_bytes = 2 * (str_len + 1) - len(encoded)  # 2 bytes a character plus null-terminator
+                    else:
+                        encoded = list(var.encode("utf-16-le"))
+                        byte_list += encoded
+                        remaining_bytes = 2 * (str_len + 1) - len(encoded)  # 2 bytes a character plus null-terminator
+                    byte_list.extend(remaining_bytes * [0])
                 elif plc_datatype not in DATATYPE_MAP:
                     raise RuntimeError("Datatype not found. Check structure definition")
                 else:

pyads/pyads_ex.py

@@ -15,7 +15,7 @@
 from contextlib import closing
 from functools import wraps
 
-from .utils import platform_is_linux, platform_is_windows, platform_is_freebsd
+from .utils import platform_is_linux, platform_is_windows, platform_is_freebsd, find_wstring_null_terminator
 from .structs import (
     AmsAddr,
     SAmsAddr,
@@ -1005,11 +1005,8 @@ def adsSumRead(
             elif data_symbols[data_name].dataType == ADST_WSTRING:
                 # find null-terminator 2 Bytes
                 a = sum_response[offset: offset + data_symbols[data_name].size]
-                for ix in range(1, len(a), 2):
-                    if (a[ix-1], a[ix]) == (0, 0):
-                        null_idx = ix - 1
-                        break
-                else:
+                null_idx = find_wstring_null_terminator(a)
+                if null_idx is None:
                     raise ValueError("No null-terminator found in buffer")
                 value = bytearray(sum_response[offset: offset + null_idx]).decode("utf-16-le")
             else:

pyads/utils.py

@@ -59,3 +59,16 @@ def decode_ads(message: bytes) -> str:
     ISO/IEC 8859-1 is supported.'
     """
     return message.decode("windows-1252").strip(" \t\n\r\0")
+
+
+def find_wstring_null_terminator(data: bytearray) -> Optional[int]:
+    """Find null-terminator in WSTRING (UTF-16) data.
+
+    :return: None if no null-terminator was found, else the index of the null-terminator
+
+    """
+    for ix in range(1, len(data), 2):
+        if (data[ix - 1], data[ix]) == (0, 0):
+            return ix - 1
+    else:
+        return None